The present invention relates generally to the field of characterizing an acoustic impedance and, more specifically, to the use of multiple pressure measurements using the same pressure transducer at different locations along a one dimensional acoustic waveguide terminated in the acoustic impedance to be characterized.
In a wide variety of applications, it is advantageous to characterize an acoustic impedance of a component of an engineering system, typically with a view toward modifying the component, or other interacting components, to eliminate an undesirable acoustic behavior. For example, anytime a flame is caused to burn in a confined space, a possibility exists that heat release dynamics of the flame will interact with an acoustic impedance of the confined space to produce a phenomenon known as combustion instability.
Combustion instability manifests itself as a sustained, self-excited pressure oscillation often of sufficient amplitude to be damaging to structural elements within the confined space. A gas turbine engine provides a typical example of an engineering system prone to damage by combustion instability. Knowledge of the acoustic impedance of a gas turbine engine combustor provides a designer an opportunity to reduce the likelihood of combustion instability by altering, for example, the combustor geometry. Whereas this disclosure emphasizes embodiments of the present invention applicable to a gas turbine engine, it will be obvious to one of ordinary skill in the art that the present invention is equally applicable to a wide variety of other engineering systems where acoustic impedance characterization is important.
Conventional techniques for characterizing an acoustic impedance of an engineering component involve: mounting the engineering component at one end of a one-dimensional acoustic waveguide; coupling a plurality of pressure transducers to the waveguide at various fixed locations along the waveguide; acoustically exciting the waveguide; collecting pressure measurement data; and reducing the pressure measurement data to produce an acoustic impedance estimate. However, such conventional techniques suffer a number of shortcomings.
First, any coupling apparatus interposed between a pressure transducer and the waveguide has a parasitic coupling impedance that affects the accuracy of the ultimate acoustic impedance estimate. For example, when characterizing an operating gas turbine engine combustor, it is often desirable to couple the pressure transducers to the waveguide through coupling tubes long enough to remove the pressure transducers to a safe ambient temperature. The parasitic coupling impedances of the coupling tubes then provide a significant source of error. In some cases, it may be possible to perform additional experiments to characterize these parasitic coupling impedances and reduce the error, but such additional experiments represent an additional cost of the technique. An opportunity exists, therefore, to improve accuracy and reduce cost by finding an acoustic impedance characterization method that is insensitive to parasitic coupling impedances without performing additional experiments.
Second, many conventional approaches to impedance estimation assume no knowledge of the mathematical relationships among the multiple pressure measurements acquired. Such approaches are essentially non-parametric approaches and are prone to yielding poorer results than parametric approaches utilizing a priori information about wave propagation in the acoustic waveguide. An opportunity exists, therefore, to further improve accuracy by finding an acoustic characterization method that exploits a priori knowledge of the wave shapes of the acoustic waveguide.
Third, the acoustic impedance estimate produced by conventional data reduction techniques is typically a xe2x80x9cpoint estimate,xe2x80x9d i.e., a single instance of an acoustic impedance measurement without regard to the statistical nature of the measurement process. When using such conventional data reduction techniques to compare acoustic impedances of two presumably different designs, a designer has no way of gauging whether any perceived variation is due to a true difference in the designs or due to inherent variability in the measurement process. An opportunity exists, therefore, to provide a more useful acoustic impedance estimate by providing additional estimates of the statistical reliability of the acoustic impedance estimate.
The opportunities described above are addressed, in one embodiment of the present invention, by an apparatus for characterizing an acoustic impedance of an engineering component acoustically coupled to an acoustic waveguide, the apparatus comprising: a pressure measurement apparatus adapted to be moved and to be disposed to measure pressure signals, the pressure signals being measured at respective ones of a plurality of predetermined locations along the acoustic waveguide; an exciter adapted to excite the acoustic waveguide with an excitation signal; a data collection module adapted to incorporate the pressure signals from the pressure measurement apparatus into a pressure signal set; a transform module adapted to transform the pressure signal set to a frequency domain set; a wave shape identifier adapted to identify a plurality of wave shape parameters from the frequency domain set; and a statistical computer adapted to compute from the frequency domain set a statistical measure for the wave shape parameters, the statistical measure being selected from the group consisting of residual variance, correlation coefficient, F-statistic, p-statistic, and confidence interval.